1. Field of the Invention
The present invention relates generally to a curing apparatus for curing a molding compound and, more particularly, to the curing apparatus for curing a semi-cured sheet molding compound (such as SMC material or TMC material) prepared from unsaturated polyester resin or epoxy resin impregnated with reinforcement such as glass fibers, carbon fibers, etc. and filler, pigment agent, thickener, inner mold release and an additive such as, for example, high temperature curing agent, by heating the sheet molding compound after the latter has been shaped by a shaping apparatus to a required shape with the fibers oriented in a required direction if necessary.
2. Description of the Prior Art
The assignee of the present invention has suggested, in JP Laid-open Patent Publication No. 5-069450 published Mar. 23, 1993 (or EP-A1-0503554 published Sep. 16, 1992; U.S. application. Ser. No. 849,662 filed Mar. 10, 1992; or Taiwan Patent Appln. No. 81101852 filed Mar. 11, 1992), a dry material molding method and a dry material of a compound material using a semi-cured, fiber-reinforced sheet molding compound such as SMC or TMC material for press-molding.
The molding apparatus referred to above comprises a plurality of rolls in combination with either a die mold or a corresponding number of die rolls positioned one above the other to define a transport passage therebetween. As the semi-cured sheet molding compound is transported through the transport passage between the rolls and the die mold or the die rolls, either the rolls or the die mold or die rolls, for example, the rolls, are moved repeatedly close towards and away from the die mold or die rolls to compress the sheet molding compound to a desired thickness so as to shape the sheet molding compound and also to orient the reinforcement fibers contained therein in a predetermined direction. The pressure applied from the rolls to the sheet molding compound to compress the latter is varied as the sheet molding compound travels through the transport passage and, for this reason, not only is control of the orientation of the reinforcement fibers contained in the sheet molding compound possible, but it is also possible to orient such reinforcement fibers to thereby avoid an arbitrary uneven flow of the material during the molding so that the molding compound having a desired or predetermined cross-sectional shape can be obtained.
The suggested molding apparatus includes a shaping apparatus comprising rolls and die rolls positioned one above the other for shaping the sheet molding compound, containing an additive such as a high temperature curing agent, at normal temperatures while the sheet molding compound is in a substantially semi-cured state. The molding apparatus also includes a heat curing apparatus for curing the shaped sheet molding compound. Thus, the suggested molding apparatus is featured in that the shaping and the curing are carried out separately but in succession, and the curing apparatus is operable merely to perform the curing.
The molding apparatus disclosed in the above mentioned prior application suggests two types of curing apparatus to be installed next to the shaping apparatus:
1) A die in the shaping apparatus is loaded into the curing apparatus while the molding compound is retained on the die, and the molding compound is cured by heating for a predetermined length of time by means of a heating means such as, for example, a heater or hot air device embedded in the molding apparatus.
2) As shown in FIG. 12 of the accompanying drawings and in JP Laid-open Patent Publication No. 5-069450 published Mar. 23, 1993 (or EP-A1-0503554 published Sep. 16, 1992; U.S. appln. Ser. No. 849,662 filed Mar. 10, 1992; or Taiwan Patent Appln. No. 81101852 filed Mar. 11, 1992), a curing apparatus 4 having a transport passage 5 of a cross-sectional shape complemental to that of the sheet molding compound 6 is installed preceding a cutting unit 7 and next to a shaping apparatus 3 comprising rolls 1 and die rolls 2 positioned one above the other. This curing apparatus 4 includes a guide zone 4A, a primary curing zone 4B, a secondary curing zone 4C and a third curing zone 4D defined therein in this order from an upstream end to a downstream end. Respective portions of the transport passage 5 in the guide and primary curing zones 4A and 4B are of a size sufficient to provide a clearance relative to an outer perimeter of the sheet molding compound defined by the shaping apparatus whereas a portion of the transport passage 5 in the third curing zone 4D is slightly undersized or slightly oversized relative to the outer perimeter of the sheet molding compound so as to accommodate a thermal characteristic of the sheet molding compound being treated.
The semi-cured sheet molding compound such as SMC material is, after having been shaped by the shaping apparatus to a desired shape, cured by heating. A series of experiments have, however, shown that, as shown in FIG. 11, during the curing treatment of the sheet molding compound, the latter tends to thermally expand and then shrink and the shrinkage will no longer take place after the sheet molding compound has been shrunken a predetermined quantity. As shown by solid, dotted and chain lines in the graph of FIG. 11, the amount of expansion and that of shrinkage vary with the type of the SMC material. In other words, the solid line indicates that the maximum amount of thermal expansion is 0.60 mm; the dotted line indicates that the maximum amount of thermal expansion is 0.49 mm; and the chain line indicates that the maximum amount of thermal expansion is 0.39 mm. Thus, the amount of thermal expansion is not uniform for all of the sheet molding compounds.
The apparatus suggested under (1) above is used to provide a product having predetermined length and cannot therefore be used for curing a long or continuous sheet molding compound.
Also, the apparatus suggested under (2) above cannot be operable where both of the coefficient of thermal expansion and the coefficient of thermal shrinkage vary since the cross-sectional shape of the transport passage through which the sheet molding compound is transported is fixed. Accordingly, in the case, for example, where the coefficient of thermal expansion is high as compared with the cross-sectional shape of the transport passage, an excessive load will act on the sheet molding compound, whereas in the case where the coefficient of thermal shrinkage is low as compared with the cross-sectional shape of the transport passage a molding face defining the transport passage will not contact the sheet molding compound resulting in not only an insufficient heating of the sheet molding compound, but also a failure to form a smooth exterior surface of the eventually cured molding compound.